The French mathematician Urbain Le Verrier,
co-predictor with J.C. Adams of the position of Neptune before it
was seen, in a lecture at 2 Jan 1860 announced that the problem
of observed deviations of the motion of Mercury could be solved
by assuming an intra-Mercurial planet, or possibly a second
asteroid belt inside Mercury's orbit. The only possible way to
observe this intra-Mercurial planet or asteroids was if/when they
transited the Sun, or during total solar eclipses. Prof. Wolf at
the Zurich sunspot data center, found a number of suspicious
"dots" on the Sun, and another astronomer found some
more. A total of two dozen spots seemed to fit the pattern of two
intra-Mercurial orbits, one with a period of 26 days and the
other of 38 days.

In 1859, Le Verrier received a letter from
the amateur astronomer Lescarbault, who reported having seen a
round black spot on the Sun on March 26 1859, looking like a
planet transiting the Sun. He had seen the spot one hour and a
quarter, when it moved a quarter of the solar diameter.
Lescarbault estimated the orbital inclination to between 5.3 and
7.3 degrees, its longitude of node about 183 deg, its
eccentricity "enormous", and its transit time across
the solar disk 4 hours 30 minutes. Le Verrier investigated this
observation, and computed an orbit from it: period 19 days 7
hours, mean distance from Sun 0.1427 a.u., inclination 12° 10',
ascending node at 12° 59' The diameter was considerably smaller
than Mercury's and its mass was estimated at 1/17 of Mercury's
mass. This was too small to account for the deviations of
Mercury's orbit, but perhaps this was the largest member of that
intra-Mercurial asteroid belt? Le Verrier fell in love with the
planet, and named it Vulcan.

In 1860 there was a total eclipse of the
Sun. Le Verrier mobilized all French and some other astronomers
to find Vulcan - nobody did. Wolf's suspicious 'sunspots' now
revived Le Verrier's interest, and just before Le Verrier's death
in 1877 some more 'evidence' found its way into print. On April 4
1875, a German astronomer, H. Weber, saw a round spot on the Sun.
Le Verrier's orbit indicated a possible transit at April 3 that
year, and Wolf noticed that his 38-day orbit also could have
performed a transit at about that time. That 'round dot' was also
photographed at Greenwich and in Madrid.

There was one more flurry after the total
solar eclipse at July 29 1878, where two observers claimed to
have seen in the vicinity of the Sun small illuminated disks
which could only be small planets inside Mercury's orbit: J.C
Watson (professor of astronomy at the Univ. of Michigan) believed
he'd found TWO intra-Mercurial planets! Lewis Swift (co-discoverer
of Comet Swift-Tuttle, which returned 1992), also saw a 'star' he
believed to be Vulcan -- but at a different position than either
of Watson's two 'intra-Mercurials'. In addition, neither Watson's
nor Swift's Vulcans could be reconciled with Le Verrier's or
Lescarbault's Vulcan. Here you can see as an example the orbit of the supposed
planet "Vulcan", which Weston theorized in 1920.

After this, nobody ever saw Vulcan again,
in spite of several searches at different total solar eclipses.
And in 1916, Albert Einstein published his General Theory of
Relativity, which explained the deviations in the motions of
Mercury without the need to invoke an unknown intra-Mercurial
planet. In May 1929 Erwin Freundlich, Potsdam, photographed the
total solar eclipse in Sumatra, and later carefully examined the
plates which showed a profusion of star images. Comparison plates
were taken six months later. No unknown object brighter than 9th
magnitude was found near the Sun.

But what did these people really see?
Lescarbault had no reason to tell a fairytale, and even Le
Verrier believed him. It is possible that Lescarbault happened to
see a small asteroid passing very close to the Earth, just inside
Earth's orbit. Such asteroids were unknown at that time, so
Lescarbault's only idea was that he saw an intra-Mercurial planet.
Swift and Watson could, during the hurry to obtain observations
during totality, have misidentified some stars, believing they
had seen Vulcan.

"Vulcan" was briefly revived
around 1970-1971, when a few researchers thought they had
detected several faint objects close to the Sun during a total
solar eclipse. These objects might have been faint comets, and
later comets have been observed that later did pass close enough
to the Sun to collide with it.

Two days before the 29 March 1974 Mariner
10 flyby past Mercury, one instument began registering bright
emissions in the extreme UV that had "no right to be there".
The next day it was gone. Three days later it reappeared, and the
"object" appeared to detach itself from Mercury. The
astronomers first thought they had seen a star. But they had seen
it in two quite different directions, and every astronomer knew
that these extreme UV wavelengths couldn't penetrate very far
through the interstellar medium, suggesting that the object must
be close. Did Mercury have a moon?

After a hectic Friday, when the "object"
had been computed to move at 4 km/s, a speed consistent with that
of a moon, JPL managers were called in. They turned the then-dying
spacecraft over full time to the UV team, and everyone started
worrying about a press conference scheduled for later that
Saturday. Should the suspected moon be announced? But the press
already knew. Some papers -- the bigger, more respectable ones --
played it straight; many others ran excited stories about
Mercury's new moon.

And the "moon" itself? It headed
straight on out from Mercury, and was eventually identified as a
hot star, 31 Crateris. What the original emissions came from, the
ones spotted on the approach to the planet, remains a mystery. So
ends the story of Mercury's moon but at the same time a new
chapter in astronomy began: extreme UV turned out not to be so
completely absorbed by the interstellar medium as formerly
believed. Already the Gum nebula has turned out to be a quite
strong emitter in the extreme UV, and spreads across 140 degrees
of the night sky at 540 angstroms. Astronomers had discovered a
new window through which to observe the heavens.

In 1672, Giovanni Domenico Cassini, one of
the prominent astronomers of the time, noticed a small companion
close to Venus. Did Venus have a satellite? Cassini decided not
to announce his observation, but 14 years later, in 1686, he saw
the object again, and then entered it in his journal. The object
was estimated to have about 1/4 the diameter of Venus, and it
showed the same phase as Venus. Later, the object was seen by
other astronomers as well: by James Short in 1740, Andreas Mayer
in 1759, J. L. Lagrange in 1761 (Lagrange announced that the
orbital plane of the satellite was perpendicular to the ecliptic).
During 1761 the object was seen a total of 18 times by five
observers. The observations of Scheuten on June 6 1761 was
especially interesting: he saw Venus in transit across the Sun's
disk, accompanied by a smaller dark spot on one side, which
followed Venus in its transit. However, Samuel Dunn at Chelsea,
England, who also watched that transit, did not see that
additional spot. In 1764 there were 8 observations by two
observers. Other observers tried to see the satellite but failed
to find it.

Now the astronomical world was faced with a
controversy: several observers had reported seeing the satellite
while several others had failed to find it in spite of determined
efforts. In 1766, the director of the Vienna observatory, Father
Hell (!), published a treatise where he declared that all
observations of the satellite were optical illusions -- the image
of Venus is so bright that it is reflected in the eye, back into
the telescope, creating a secondary image at a smaller scale.
Others published treatises declaring that the observations were
real. J. H. Lambert of Germany published orbital elements of the
satellite in Berliner Astronomischer Jahrbuch 1777: mean distance
66.5 Venus radii, orbital period 11 days 3 hours, inclination to
ecliptic 64 degrees. It was hoped that the satellite could be
seen during the transit of Venus in front of the Sun June 1 1777
(it is self evident that Lambert made a mistake in calculating
these orbital elements: at 66.5 Venus radii, the distance from
Venus is about the same as our Moon's distance from the Earth.
This fits very bad with the orbital period of 11 days or only
somewhat more than 1/3 of the orbital period of our Moon. The
mass of Venus is a little smaller than the mass of the Earth).

In 1768 there was one more observation of
the satellite, by Christian Horrebow in Copenhagen. There were
also three searches, one made by one of the greatest astronomers
of all time, William Herschel -- all three of them failed to find
any satellite. Quite late in the game, F. Schorr from Germany
tried to make a case for the satellite in a book published in
1875.

In 1884, M. Hozeau, former director of the
Royal Observatory of Brussels, suggested a different hypothesis.
By analysing available observations Hozeau concluded that Venus
moon appeared close to Venus approximately every 2.96 years or
1080 days. Hozeau suggested that it wasn't a moon of Venus, but a
planet of its own, orbiting the sun once every 283 days and thus
being in conjunction with Venus once every 1080 days. Hozeau also
named it Neith, after the mysterious goddess of Sais, whose veil
no mortal raised.

In 1887, three years after the "moon
of Venus" had been revived by Hozeau, the Belgian Academy of
Sciences published a long paper where each and every reported
observation was investigated in detail. Several observations of
the satellite were really stars seen in the vicinity of Venus.
Roedkier's observations "checked out" especially well
-- he had been fooled, in succession, by Chi Orionis, M Tauri, 71
Orionis, and Nu Geminorum! James Short had really seen a star
somewhat fainter than 8th magnitude. All observations by Le
Verrier and Montaigne could be similarly explained. Lambert's
orbital calculations were demolished. The very last observation,
by Horrebow in 1768, could be ascribed to Theta Librae.

After this paper was published, only one
more observation was reported, by a man who had earlier made a
search for the satellite of Venus but failed to find it: on Aug
13 1892, E. E. Barnard recorded a 7th magnitude object near Venus.
There is no star in the position recorded by Barnard, and
Barnard's eyesight was notoriously excellent. We still don't know
what he saw. Was it an asteroid that hadn't been charted? Or was
it a short-lived nova that nobody else happened to see?

In 1846, Frederic Petit, director of the
observatory of Toulouse, stated that a second moon of the Earth
had been discovered. It had been seen by two observers, Lebon and
Dassier, at Toulouse and by a third, Lariviere, at Artenac,
during the early evening of March 21 1846. Petit found that the
orbit was elliptical, with a period of 2 hours 44 minutes 59
seconds, an apogee at 3570 km above the Earth's surface and
perigee at just 11.4 km (!) above the Earth's surface. Le
Verrier, who was in the audience, grumbled that one needed to
take air resistance into account, something nobody could do at
that time. Petit became obsessed with this idea of a second moon,
and 15 years later announced that he had made calculations about
a small moon of Earth which caused some then-unexplained
peculiarities in the motion of our main Moon. Astronomers
generally ignored this, and the idea would have been forgotten if
not a young French writer, Jules Verne, had not read an abstract.
In Verne's novel "From the Earth to the Moon", Verne
lets a small object pass close to the traveller's space capsule,
causing it to travel around the Moon instead of smashing into it:

"It is", said Barbicane,
"a simple meteorite but an enormous one, retained as a
satellite by the attraction of the Earth."

"Is that possible?",
exclaimed Michel Ardan, "the earth has two moons?"

"Yes, my friend, it has two moons,
although it is usually believed to have only one. But this
second moon is so small and its velocity is so great that the
inhabitants of Earth cannot see it. It was by noticing
disturbances that a French astronomer, Monsieur Petit, could
determine the existence of this second moon and calculated
its orbit. According to him a complete revolution around the
Earth takes three hours and twenty minutes. . . . "

"Do all astronomers admit the the
existence of this satellite?", asked Nicholl

"No", replied Barbicane,
"but if, like us, they had met it they could no longer
doubt it. . . . But this gives us a means of determining our
position in space . . . its distance is known and we were,
therefore, 7480 km above the surface of the globe where we
met it."

Jules Verne was read by millions of people,
but not until 1942 did anybody notice the discrepancies in
Verne's text:

A satellite 7480 km above the Earth's
surface would have a period of 4 hours 48 minutes, not 3
hours 20 minutes.

Since it was seen from the window from
which the Moon was invisible, while both were
approaching, it must be in retrogade motion, which would
be worth remarking. Verne doesn't mention this.

In any case the satellite would be in
eclipse and thus be invisible. The projectile doesn't
leave the Earth's shadow until much later.

Nevertheless, Jules Verne made Petit's
second moon known all over the world. Amateur astronomers jumped
to the conclusion that here was opportunity for fame -- anybody
discovering this second moon would have his name inscribed in the
annals of science. No major observatory ever checked the problem
of the Earth's second moon, or if they did they kept quiet.
German amateurs were chasing what they called Kleinchen
("little bit") -- of course they never found Kleinchen.

W. H. Pickering devoted his attention to
the theory of the subject: if the satellite orbited 320 km above
the surface and if its diameter was 0.3 meters, with the same
reflecting power as the Moon, it should be visible in a 3-inch
telescope. A 3 meter satellite would be a naked-eye object of
magnitude 5. Though Pickering did not look for the Petit object,
he did carry on a search for a secondary moon -- a satellite of
our Moon ("On a photographic search for a satellite of the
Moon", Popular Astronomy, 1903). The result was negative and
Pickering concluded that any satellite of our Moon must be
smaller than about 3 meters.

Pickering's article on the possibility of a
tiny second moon of Earth, "A Meteoritic Satellite",
appeared in Popular Astronomy in 1922 and caused another short
flurry among amateur astronomers, since it contained a virtual
request: "A 3-5-inch telescope with a low-power eyepiece
would be the likeliest mean to find it. It is an opportunity for
the amateur." But again, all searches remained fruitless.

The original idea was that the
gravitational field of the second moon should account for the
then inexplicable minor deviations of the motion of our big Moon.
That meant an object at least several miles large -- but if such
a large second moon really existed, it would have been seen by
the Babylonians. Even if it was too small to show a disk, its
comparative nearness would have made it move fast and therefore
be conspicuous, as today's watchers of artificial satellites and
even airplanes know. On the other hand, nobody was much
interested in moonlets too small to be seen.

There have been other proposals for
additional natural satellites of the Earth. In 1898 Dr Georg
Waltemath from Hamburg claimed to have discovered not only a
second moon but a whole system of midget moons. Waltemath gave
orbital elements for one of these moons: distance from Earth 1.03
million km, diameter 700 km, orbital period 119 days, synodic
period 177 days. "Sometimes", says Waltemath, "it
shines at night like the Sun" and he thinks this moon was
seen in Greenland on 24 October 1881 by Lieut Greely, ten days
after the Sun had set for the winter. Public interest was aroused
when Waltemath predicted his second moon would pass in front of
the Sun on the 2nd, 3rd or 4th of February 1898. On the 4th
February, 12 persons at the post office of Greifswald (Herr
Postdirektor Ziegel, members of his family, and postal employees)
observed the Sun with their naked eye, without protection of the
glare. It is easy to imagine a faintly preposterous scene: an
imposing-looking Prussian civil servant pointing skyward through
his office window, while he reads Waltemath's prediction aloud to
a know of respectful subordinates. On being interviewed, these
witnesses spoke of a dark object having one fifth the Sun's
apparent diameter, and which took from 1:10 to 2:10 Berlin time
to traverse the solar disk. It was soon proven to be a mistake,
because during that very hour the Sun was being scrutinized by
two experienced astronomers, W. Winkler in Jena and Baron Ivo von
Benko from Pola, Austria. They both reported that only a few
ordinary sunspots were on the disk. The failure of this and later
forecasts did not discourage Waltemath, who continued to issue
predicitons and ask for verifications. Contemporary astronomers
were pretty irritated over and over again having to answer
questions from the public like "Oh, by the way, what about
all these new moons?". But astrologers caught on -- in 1918
the astrologer Sepharial named this moon Lilith. He
considered it to be black enough to be invisible most of the
time, being visible only close to opposition or when in transit
across the solar disk. Sepharial constructed an ephemeris of
Lilith, based on several of Waltemath's claimed observations. He
considered Lilith to have about the same mass as the Moon,
apparently happily unaware that any such satellite would, even if
invisible, show its existence by perturbing the motion of the
Earth. And even to this day, "the dark moon" Lilith is
used by some astrologers in their horoscopes.

From time to time other "additional
moons" were reported from observers. The German astronomical
magazine "Die Sterne" reported that a German amateur
astronomer named W. Spill had observed a second moon cross our
first moon's disc on May 24, 1926.

Around 1950, when artificial satellites
began to be discussed in earnest, everybody expected them to be
just burned-out upper stages of multistage rockets, carrying no
radio transmitters but being tracked by radar from the Earth. In
such cases a bunch of small nearby natural satellites would have
been most annoying, reflecting radar beams meant for the
artificial satellites. The method to search for such natural
satellites was developed by Clyde Tombaugh: the motion of a
satellite at e.g. 5000 km height is computed. Then a camera
platform is constructed that scans the sky at precisely that rate.
Stars, planets etc will then appear as lines on the photographs
taken by this camera, while any satellite at the correct altitude
will appear as a dot. If the satellite was at a somewhat
different altitude, it would produce a short line.

Observations began in 1953 at the Lowell
Observatory and actually invaded virgin territory: with the
exception of the Germans searching for "Kleinchen"
nobody had ever paid attention to the space between the Moon and
the Earth! By the fall of 1954, weekly journals and daily
newspapers of high reputation stated that the search had brought
its first results: one small natural satellite at 700 km
altitude, another one 1000 km out. One general is said to have
asked: "Is he sure they're natural?". Nobody seems to
know how these reports originated -- the searches were completely
negative. When the first artificial satellites were launched in
1957 and 1958, the cameras tracked those satellites instead.

But strangely enough, this does not mean
that the Earth only has one natural satellite. The Earth can have
a very near satellite for a short time. Meteoroids passing the
Earth and skimming through the upper atmosphere can lose enough
velocity to go into a satellite orbit around the Earth. But since
it passes the upper atmosphere at each perigee, they will not
last long, maybe only one or two, possibly a hundred revolutions
(about 150 hours). There are some indications that such "ephemeral
satellites" have been seen; it is even possible that Petit's
observers did see one.

In addition to ephemeral satellites there
are two more possibilities. One is that the Moon had a satellite
of its own -- but despite several searches none has been found (in
addition it's now known that the gravity field of the Moon is
uneven or "lumpy" enough for any lunar satellite orbit
to be unstable -- any lunar satellite will therefore crash into
the Moon after a fairly short time, a few years or possibly a
decade). The other possibility is that there might be Trojan
satellites, i.e. secondary satellites in the lunar orbit,
travelling 60 degrees ahead of or behind the Moon.

Such "Trojan satellites" were
first reported by the Polish astronomer Kordylewski of Krakow
observatory. He started his search in 1951, visually with a good
telescope. He was hoping for reasonably large bodies in the lunar
orbit, 60 degrees away from the Moon. The search was negative,
but in 1956 his compatriot and colleague, Wilkowski, suggested
that there may be many tiny bodies, too small to be seen
individually but many enough to appear as a cloud of dust
particles. In such a case, they would be best visible without a
telescope i.e. with the naked eye! Using a telescope would "magnify
it out of existence". Dr Kordylewski was willing to try. A
dark night with clear skies, and the Moon being below the
horizon, was required.

In October 1956, Kordylewski saw, for the
first time, a fairly bright patch in one of the two positions. It
was not small, subtending an angle of 2 degrees (i.e. about 4
times larger than the Moon itself), and was very faint, only
about half as bright as the notoriously difficult Gegenschein (counterglow
-- a bright patch in the zodiacal light, directly opposite to the
Sun). In March and April 1961, Kordylewski succeeded in
photographing two clouds near the expected positions. They seem
to vary in extent, but that may be due to changing illumination.
J. Roach detected these cloud satellites in 1975 with the OSO (Orbiting
Solar Observatory) 6 spacecraft. In 1990 they were again
photographed, this time by the polish astronomer Winiarski, who
found that they were a few degrees in apparent diameter, that
they "wandered" up to ten degrees away from the "trojan"
point, and that they were somewhat redder than the zodiacal light.

So the century-long search for a second
moon of the Earth seems to have succeeded, after all, even though
this 'second moon' turned out to be entirely different from
anything anybody had ever expected. They are very hard to detect
and to distinguish from the zodiacal light, in particular the
Gegenschein.

But people are still proposing additional
natural satellites of the Earth. Between 1966 and 1969 John
Bargby, an American scientist, claimed to have observed at least
ten small natural satellites of the Earth, visible only in a
telescope. Bargby found elliptical orbits for all the objects:
eccentricity 0.498, semimajor axis 14065 km, which yields perigee
and apogee heights of 680 and 14700 km. Bargby considered them to
be fragments of a larger body which broke up in December 1955. He
based much of his suggested satellites on supposed perturbations
of artificial satellites. Bargby used artificial satellite data
from Goddard Satellite Situation Report, unaware that the values
in this publication are only approximate and sometimes grossly in
error and can therefore not be used for any precise scientific
analysis. In addition, from Bargby's own claimed observations it
can be deduced that when at perigee Bargby's satellites ought to
be visible at first magnitude and thus be easily visible to the
naked eye, yet no-one has seen them as such.

The first to guess that Mars had moons was
Johannes Kepler in 1610. When trying to solve Galileo's anagram
referring to Saturn's rings, Kepler thought that Galileo had
found moons of Mars instead.

In 1643, the Capuchin monk Anton Maria
Shyrl claimed to really have seen the moons of Mars. We now know
that would be impossible with the telescopes of that time --
probably Shyrl got deceived by a star nearby Mars.

In 1727, Jonathan Swift in "Gulliver's
Travels" wrote about two small moons orbiting Mars, known to
the Lilliputian astronomers. Their periods of revolution were 10
and 21.5 hours. These 'moons' were in 1750 adopted by Voltaire in
his novel "Micromegas", the story of a giant from
Sirius visiting our solar system.

In 1747 a German captain, Kindermann, had
claimed to have seen the moon (just one!) of Mars, on 10 July
1744. Kindermann reported the orbital period of this martian moon
as 59 hours 50 minutes and 6 seconds (!)

In 1877, Asaph Hall finally discovered
Phobos and Deimos, the two small moons of Mars. Their orbital
periods are 7 hours 39 minutes amd 30 hours 18 minutes, quite
close to the periods guessed by Jonathan Swift 150 years earlier!

In 1975, Charles Kowal at Palomar (discoverer
of Comet 95 P/Chiron) photographed an object thought to be a new
satellite of Jupiter. It was seen several times, but not enough
to determine an orbit, then lost. It used to show up as a
footnote in texts of the late 70s.

In April 1861 Hermann Goldschmidt announced
the discovery of a 9th moon of Saturn, which orbited the planet
between Titan and Hyperion. He named that moon Chiron (!).
However the discovery was never confirmed -- nobody else ever saw
this satellite "Chiron". Later, Pickering discovered
what's now considered Saturn's 9th moon, Phoebe, in 1898. This
was the first time a satellite of another planet was discovered
by photographical observations. Phoebe is also Saturn's outermost
moon.

In 1905, Pickering though he had discovered
a tenth moon, which he named Themis. According to
Pickering, it orbited Saturn between the orbits of Titan and
Hyperion in a highly inclined orbit: mean distance from Saturn 1,460,000
km, orbital period 20.85 days, eccentricity 0.23, inclination 39
degrees. Themis was never seen again, but nevertheless appeared
in almanacs and astronomy books well into the 1950's and 1960's.

In 1966, A. Dollfus discovered another new
moon of Saturn. It was named Janus, and orbited Saturn just
outside its rings. It was so faint and close to the rings that
the only chance to see it was when the rings of Saturn were seen
from the edge, as happened in 1966. Now Janus was Saturn's tenth
moon.

In 1980, when Saturns rings again were seen
edgewise, a flurry of observations discovered a lot of new
satellites close to the rings of Saturn. Close to Janus another
satellite was discovered, named Epimetheus. Their orbits are very
close to each other, and the most interesting aspect of this
satellite pair is that they regularly switch orbits with each
other! It turned out that the "Janus" discovered in
1966 really were observations of both of these co-orbital
satellites. Thus the 'tenth moon of Saturn' discovered in 1966
really turned out to be two different moons! The spacecraft
Voyager 1 and Voyager 2, which travelled past Saturn shortly
afterwards, confirmed this.

In 1787, William Herschel announced the
discovery of six satellites of Uranus. Herschel here made a
mistake -- only two of his six satellites were real (Titania and
Oberon, the largest and outermost two satellites), the remaining
four were just stars which happened to be nearby (...I think I've
heard this story before.... :-)

In 1841, John Couch Adams began
investigating the by then quite large residuals in the motion of
Uranus. In 1845, Urbain Le Verrier started to investigate them,
too. Adams presented two different solutions to the problem,
assuming that the deviations were caused by the gravitation from
an unknown planet. Adams tried to present his solutions to the
Greenwich observatory, but since he was young and unknown, he
wasn't taken seriously. Urbain Le Verrier presented his solution
in 1846, but France lacked the necessary resources to locate the
planet. Le Verrier then instead turned to the Berlin observatory,
where Galle and his assistant d'Arrest found Neptune on the
evening of Sept 23, 1846. Nowadays, both Adams and Le Verrier
share the credit of having predicted the existence and position
of Neptune.

(Inspired by this success, Le Verrier
attacked the problem of the deviations of Mercury's orbit, and
suggested the existence of an intra-mercurial planet, Vulcan, which later turned out to be non-existent.)

On 30 Sept 1846, one week after the
discovery of Neptune, Le Verrier declared that there may be still
another unknown planet out there. On October 10, Neptune's large
moon Triton was discovered, which yielded an easy way to
accurately determine the mass of Neptune, which turned out to be
2% larger than expected from the perturbations upon Uranus. It
seemed as if the deviations in Uranus's motion really was caused
by two planets -- in addition the real orbit of Neptune turned
out to be significantly different from the orbits predicted by
both Adams and Le Verrier.

In 1850 Ferguson was observing the motion
of the minor planet Hygeia. One reader of Ferguson's report was
Hind, who checked the reference stars used by Ferguson. Hind was
unable to find one of Ferguson's reference stars. Maury, at the
Naval Observatory, was also unable to find that star. During a
few years it was believed that this was an observation of yet
another planet, but in 1879 another explanation was offered:
Ferguson had made a mistake when recording his observation --
when that mistake was corrected, another star nicely fit his
'missing reference star'.

The first serious attempt to find a trans-Neptunian
planet was done in 1877 by David Todd. He used a "graphical
method", and despite the inconclusivenesses of the residuals
of Uranus, he derived elements for a trans-Neptunian planet: mean
distance 52 a.u., period 375 years, magnitude fainter than 13.
Its longitude for 1877.84 was given 170 degrees with an
uncertainty of 10 degrees. The inclination was 1.40 degrees and
the longitude of the ascending node 103 degrees.

In 1879, Camille Flammarion added another
hint as to the existence of a planet beyond Neptune: the aphelia
of periodic comets tend to cluster around the orbits of major
planets. Jupiter has the greatest share of such comets, and
Saturn, Uranus and Neptune also have a few each. Flammarion found
two comets, 1862 III with a period of 120 years and aphelion at
47.6 a.u., and 1889 II, with a somewhat longer period and
aphelion at 49.8 a.u. Flammarion suggested that the hypothetical
planet probably moved at 45 a.u.

One year later, in 1880, professor Forbes
published a memoir concering the aphelia of comets and their
association with planetary orbits. By about 1900 five comets were
known with aphelia outside Neptune's orbit, and then Forbes
suggested one trans-Neptunian moved at a distance of about 100 a.u.,
and another one at 300 a.u., with periods of 1000 and 5000 years.

During the next five years, several
astronomers/mathematicians published their own ideas of what
might be found in the outer parts of the solar system. Gaillot at
Paris Observatory assumed two trans-Neptunian planets at 45 and
60 a.u. Thomas Jefferson Jackson See predicted three trans-Neptunian
planets: "Oceanus" at 41.25 a.u. and period 272 years,
"trans-Oceanus" at 56 a.u. and period 420 years, and
finally another one at 72 a.u. and period 610 years. Dr Theodor
Grigull of Munster, Germany, assumed in 1902 a Uranus-sized
planet at 50 a.u. and period 360 years, which he called "Hades".
Grigull based his work mainly on the orbits of comets with
aphelia beyond Neptune's orbit, with a cross check whether the
gravitational pull of such a body would produce the observed
deviations in Uranus motion. In 1921 Grigull revised the orbital
period of "Hades" to 310-330 years, to better fit the
observed deviations.

In 1900 Hans-Emil Lau, Copenhagen,
published elements of two trans-Neptunian planets at 46.6 and 70.7
a.u. distance, with masses of 9 and 47.2 times the Earth, and a
magnitude for the nearer planet around 10-11. The 1900 longitudes
of those hypothetical bodies were 274 and 343 degrees, both with
the very large uncertainty of 180 degrees.

In 1901, Gabriel Dallet deduced a
hypothetical planet at 47 a.u. with a magnitude of 9.5-10.5 and a
1900 longitude of 358 degrees. The same year Theodor Grigull
derived a longitude of a trans-Neptunian planet less than 6
degrees away from Dallet's planet, and later brought the
difference down to 2.5 degrees. This planet was supposed to be 50.6
a.u. distant.

In 1904, Thomas Jefferson Jackson See
suggested three trans-Neptunian planets, at 42.25, 56 and 72 a.u.
The inner planet had a period of 272.2 years and a longitude in
1904 of 200 degrees. A Russian general named Alexander Garnowsky
suggested four hypothetical planets but failed to supply any
details about them.

The two most carefully worked out
predictions for the Trans-Neptune were both of American origin:
Pickering's "A search for a planet beyond Neptune" (Annals
Astron. Obs. Harvard Coll, vol LXI part II 1909), and Percival
Lowell's "Memoir on a trans-Neptunian planet" (Lynn,
Mass 1915). They were concerned with the same subject but used
different approaches and arrived at different results.

Pickering used a graphical analysis and
suggested a "Planet O" at 51.9 a.u. with a period of
373.5 years, a mass twice the Earth's and a magnitude of 11.5-14.
Pickering suggested eight other trans-Neptunian planets during
the forthcoming 24 years. Pickerings results caused Gaillot to
revise the distances of his two trans-Neptunians to 44 and 66 a.u.,
and he gave them masses of 5 and 24 Earth masses.

All in all, from 1908 to 1932, Pickering
proposed seven hypothetical planets -- O, P, Q, R, S, T and U.
His final elements for O and P define completely different bodies
than the orginal ones, so the total can be set at nine, certainly
the record for planetary prognostication. Most of Pickerings
predictions are only of passing interest as curiosities. In 1911
Pickering suggested that planet Q had a mass of 20,000 Earths,
making it 63 times more massive than Jupiter or about 1/6 the
Sun's mass, close to a star of minimal mass. Pickering said
planet Q had a highly elliptical orbit.

In later years only planet P seriously
occupied his attention. In 1928 he reduced the disance of P from
123 to 67.7 a.u., and its period from 1400 to 556.6 years. He
gave P a mass of 20 Earth masses and a magnitude of 11. In 1931,
after the discovery of Pluto, he issued another elliptical orbit
for P: distance 75.5 a.u., period 656 years, mass 50 Earth
masses, eccentricity 0.265, inclination 37 degrees, close to the
values given for the 1911 orbit. His Planet S, proposed in 1928
and given elements in 1931, was put at 48.3 a.u. distance (close
to Lowell's Planet X at 47.5 a.u.), period 336 years, mass 5
Earths, magnitude 15. In 1929 Pickering proposed planet U,
distance 5.79 a.u., period 13.93 years, i.e. barely outside
Jupiter's orbit. Its mass was 0.045 Earth masses, eccentricity 0.26.
The least of Pickering's planets is planet T, suggested in 1931:
distance 32.8 a.u., period 188 years.

Percival Lowell, most well known as a
proponent for canals on Mars, built a private observatory in
Flagstaff, Arizona. Lowell called his hypothetical planet Planet X, and performed several searches for it, without
success. Lowell's first search for Planet X came to an end in
1909, but in 1913 he started a second search, with a new
prediction of Planet X: epoch 1850-01-01, mean long 11.67 deg,
perih. long 186, eccentricity 0.228, mean dist 47.5 a.u. long arc
node 110.99 deg, inclination 7.30 deg, mass 1/21000 solar masses.
Lowell and others searched in vain for this Planet X in 1913-1915.
In 1915, Lowell published his theoretical results of Planet X. It
is ironical that this very same year, 1915, two faint images of
Pluto was recorded at Lowell observatory, although they were
never recognized as such until after the discovery of Pluto (1930).
Lowell's failure of finding Planet X was his greatest
disappointment in life. He didn't spend much time looking for
Planet X during the last two years of his life. Lowell died in
1916. On the nearly 1000 plates exposed in this second search
were 515 asteroids, 700 variable stars and 2 images of Pluto!

The third search for Planet X began in
April 1927. No progress was made in 1927-1928. In December 1929 a
young farmer's boy and amateur astronomer, Clyde Tombaugh from
Kansas, was hired to do the search. Tombaugh started his work in
April 1929. On January 23 and 29, Tombaugh exposed the pair of
plates on which he found Pluto when examining them on February 18.
By then Tombaugh had examined hundreds of plate pairs and
millions of stars. The search for Planet X had come to an end.

Or had it? The new planet, later named
Pluto, turned out to be disappointingly small, perhaps only one
Earth mass put probably only about 1/10 Earth masses or smaller (in
1979, when Pluto's satellite Charon was discovered, the mass of
the Pluto-Charon pair turned out to be only about 1/1000 Earth
mass!). Planet X must, if it was causing those perturbations in
the orbit of Uranus, be much larger than that! Tombaugh continued
his search another 13 years, and examined the sky from the north
celestial pole to 50 deg. south declination, down to magnitude 16-17,
sometimes even 18. Tombaugh examined some 90 million images of
some 30 million stars over more than 30,000 square degrees on the
sky. He found one new globular cluster, 5 new open star clusters,
one new supercluster of 1800 galaxies and several new small
galaxy clusters, one new comet, about 775 new asteroids -- but no
new planet except Pluto. Tombaugh concluded that no unknown
planet brighter than magnitude 16.5 did exist -- only a planet in
an almost polar orbit and situated near the south celestial pole
could have escaped his detection. He could have picked up a
Neptune-sized planet at seven times the distance of Pluto, or a
Pluto-sized planet out to 60 a.u.

The naming of Pluto is a story by itself. Early suggestions of the name
of the new planet were: Atlas, Zymal, Artemis, Perseus, Vulcan,
Tantalus, Idana, Cronus. The New York Times suggested Minerva,
reporters suggested Osiris, Bacchus, Apollo, Erebus. Lowell's
widow suggested Zeus, but later changed her mind to Constance.
Many people suggested the planet be named Lowell. The staff of
the Flagstaff observatory, where Pluto was discovered, suggested
Cronus, Minerva, and Pluto. A few months later the planet was
officially named Pluto. The name Pluto was originally suggested
by Venetia Burney, an 11-year-old schoolgirl in Oxford, England.

The very first orbit computed for Pluto
yielded an eccentricity of 0.909 and a period of 3000 years! This
cast some doubt whether it was a planet or not. However, a few
months later, considerably better orbital elements for Pluto was
obtained. Below is a comparison of the orbital elements of
Lowell's Planet X, Pickering's Planet O, and Pluto:

The mass of Pluto was very hard to
determine. Several values were given at different times -- the
matter wasn't settled until James W. Christy discovered Pluto's
moon Charon in June 1978 -- Pluto was then shown to have only 20%
of the mass of our Moon! That made Pluto hopelessely inadequate
to produce measureable gravitational perturbations on Uranus and
Neptune. Pluto could not be Lowell's Planet X -- the planet found
was not the planet sought. What seemed to be another triumph of
celestial mechanics turned out to be an accident -- or rather a
result of the intelligence and thoroughness of Clyde Tombaugh's
search.

Another short-lived trans-Neptunian suspect
was reported on April 22 1930 by R.M. Stewart in Ottawa, Canada
-- it was reported from plates taken in 1924. Crommelin computed
an orbit (dist 39.82 a.u., asc node 280.49 deg, inclination 49.7
deg!). Tombaugh searched for the "Ottawa object"
without finding it. Several other searhes were made, but nothing
was ever found.

Meanwhile Pickering continued to predict
new planets (see above). Others also predicted new planets on
theoretical grounds (Lowell himself had already suggested a
second trans-Neptunian at about 75 a.u.). In 1946, Francis M. E.
Sevin suggested a trans-Plutonian planet at 78 a.u. He first
derived this from a curious empirical method where he grouped the
planets and the erratic asteroid Hidalgo, into two groups of
inner and outer bodies:

He then added the logarithms of the periods
of each pair of planets, finding a roughly constant sum of about
7.34. Assuming this sum to be valid for Mercury and the trans-Plutonian
too, he arrived at a period of about 677 years for "Transpluto".
Later Sevin worked out a full set of elements for "Transpluto": dist 77.8 a.u., period 685.8 years,
eccentricity 0.3, mass 11.6 Earth masses. His prediction stirred
little interest among astronomers.

In 1950, K. Schutte of Munich used data
from eight periodic comets to suggest a trans-Plutonian planet at
77 a.u. Four years later, H. H. Kitzinger of Karlsruhe, using the
same eight comets, extended and refined the work, finding the
supposed planet to be at 65 a.u., with a period of 523.5 years,
an orbital inclination of 56 degrees, and an estimated magnitude
of 11. In 1957, Kitzinger reworked the problem and arrived at new
elements: dist 75.1 a.u., period 650 years, inclination 40
degrees, magnitude around 10. After unsuccessful photographic
searches, he re-worked the problem once again in 1959, arriving
at a mean dist of 77 a.u., period 675.7 years, inclination 38
degrees, eccentricity 0.07, a planet not unlike Sevin's "Transpluto"
and in some ways similar to Pickering's final Planet P. No such
planet has ever been found, though. Here you find a plot
of the orbit of Kitzinger's Transpluto.

Halley's Comet has also been used as a
"probe" for trans-plutonian planets. In 1942 R. S.
Richardson found that an Earth-sized planet at 36.2 a.u., or 1 a.u.
beyond Halley's aphelion, would delay Halley's perihelion passage
so that it agreed better with observations. A planet at 35.3 a.u.
of 0.1 Earth masses would have a similar effect. In 1972, Brady
predicted a planet at 59.9 a.u., period 464 years, eccentricity 0.07,
inclination 120 degrees (i.e. being in a retrograde orbit),
magnitude 13-14, size about Saturn's size. Such a trans-Plutonian planet would reduce the residuals of Halley's Comet
significantly back to the 1456 perihelium passage. This gigantic
trans-Plutonian planet was also searched for, but never found.

Tom van Flandern examined the positions of
Uranus and Neptune in the 1970s. The calculated orbit of Neptune
fit observations only for a few years, and then started to drift
away. Uranus orbit fit the observations during one revolution but
not during the previous revolution. In 1976 Tom van Flandern
became convinced that there was a tenth planet. After the
discovery of Charon in 1978 showed the mass of Pluto to be much
smaller than expected, van Flandern convinced his USNO colleague
Robert S. Harrington of the existence of this tenth planet. They
started to collaborate by investigate the Neptunian satellite
system. Soon their views diverged. van Flandern thought the tenth
planet had formed beyond Neptune's orbit, while Harrington
believed it had formed between the orbits of Uranus and Neptune.
van Flandern thought more data was needed, such as an improved
mass for Neptune furnished by Voyager 2. Harrington started to
search for the planet by brute force -- he started in 1979, and
by 1987 he had still not found any planet. van Flandern and
Harrington suggested that the tenth planet might be near aphelion
in a highly elliptical orbit. If the planet is dark, it might be
as faint as magnitude 16-17, suggests van Flandern.

In 1987, Whitmire and Matese suggested a
tenth planet at 80 a.u. with a period of 700 years and an
inclination of perhaps 45 degrees, as an alternative to their
"Nemesis" hypothesis. However, according to Eugene M.
Shoemaker, this planet could not have caused those meteor showers
that Whitmire and Matese suggested (see below).

In 1987, John Anderson at JPL examined the
motions of the spacecraft Pioneer 10 and Pioneer 11, to see if
any deflection due to unknown gravity forces could be found. None
was found -- from this Anderson concluded that a tenth planet
most likely exists! JPL had excluded observations of Uranus prior
to 1910 in their ephemerides, while Anderson had confidence in
the earlier observations as well. Anderson concluded that the
tenth planet must have a highly elliptical orbit, carrying it far
away to be undetectable now but periodically bringing it close
enough to leave its disturbing signature on the paths of the
outer planets. He suggests a mass of five Earth masses, an
orbital period of about 700-1000 years, and a highly inclined
orbit. Its perturbations on the outer planets won't be detected
again until 2600. Anderson hoped that the two Voyagers would help
to pin down the location of this planet.

Conley Powell, from JPL, also analyzed the
planetary motions. He also found that the observations of Uranus
suddenly did fit the calculations much better after 1910 than
before. Powell suggested a planet with 2.9 Earth masses at 60.8 a.u.
from the Sun, a period of 494 years, inclination 8.3 degrees and
only a small eccentricity. Powell was intrigued that the period
was approximately twice Pluto's and three times Neptune's period,
suggesting that the planet he thought he saw in the data had an
orbit stabilized by mutual resonance with its nearest neighbours
despite their vast separation. The solution called for the planet
to be in Gemini, and also being brighter than Pluto when it was
discovered. A search was performed in 1987 at Lowell Observatory
for Powell's planet -- nothing was found. Powell re-examined his
solution and revised the elements: 0.87 Earth masses, distance 39.8
a.u., period 251 years, eccentricity 0.26, i.e. an orbit very
similar to Pluto's! Currently, Powell's new planet should
be in Leo, at magnitude 12, however Powell thinks it's premature
to search for it, he needs to examine his data further.

Even if no trans-Plutonian planet ever was
found, the interest was focused to the outer parts of the solar
system. The erratic asteroid Hidalgo, moving in an
orbit between Mars and Saturn, has already been mentioned. In
1977-1984 Charles Kowal performed a new systematic search for
undiscovered bodies in the solar system, using Palomar
Observatory's 48-inch Schmidt telescope. In October 1977 he found
the asteroid 1977 UB, later named Chiron, moving at mean
distance 13.7 a.u., period 50.7 years, eccentricity 0.3786,
inclination 6.923 deg, diameter about 180 km. During his search,
Kowal also found 5 comets and 15 asteroids, including Chiron, the most distant asteroid known when it was
discovered. Kowal also recovered 4 lost comets and one lost
asteroid. Kowal did not find a tenth planet, and concluded that
there was no unknown planet brighter than 20th magnitude within 3
degrees of the ecliptic.

Chiron was first announced as a "tenth
planet", but was immediately designated as an asteroid. But
Kowal suspected it may be very comet-like, and later it has even
developed a short cometary tail! In 1995 Chiron was also
classified as a comet - it is certainly the largest comet we know
about.

In 1992 an even more distant asteroid was
found: Pholus. Later in 1992 an asteroid outside Pluto's orbit
was found, followed by five additional trans-Plutonian asteroids
in 1993 and at least a dozen in 1994!

Meanwhile, the spacecraft Pioneer 10 and 11
and Voyagers 1 and 2 had travelled outside the solar system, and
could also be used as "probes" for unknown
gravitational forces possibly from unknown planets -- nothing has
been found. The Voyagers also yielded more accurate masses for
the outer planets -- when these updated masses were inserted in
the numerical integrations of the solar system, the residuals in
the positions of the outer planets finally disappeared. It seems
like the search for "Planet X" finally has come to an
end. There was no "Planet X" (Pluto doesn't really
count), but instead an asteroid belt outside Neptune/Pluto was
found! The asteroids outside Jupiter's orbit that were known in
August 1993 are as follows:

Note by Robert von Heeren: For a actual complete list of all 67 TNOs click here!

The trans-Neptunian bodies seem to form two
groups. One group, composed e.g. of Pluto, 1993 SC, 1993
SB and 1993 RO, have eccentric orbits and a 3:2 resonance with
Neptune. The second group, including e.g. 1992 QB1 and 1993 FW, is slightly further out and in rather low
eccentricity.

Note by Robert von Heeren: You find a
more detailed description of the discovery history of the Kuiper-belt
and the different groups at my astrology page, topic "Schwerpunkt I: Neue Planeten/New Planets" in german, see for the several english articles.

Suppose our Sun was not alone but had a
companion star. Suppose that this companion star moved in an
elliptical orbit, its solar distance varying between 90,000 a.u.
(1.4 light years) and 20,000 a.u., with a period of 30 million
years. Also suppose this star is dark or at least very faint, and
because of that we haven't noticed it yet.

This would mean that once every 30 million
years that hypothetical companion star of the Sun would pass
through the Oort cloud (a hypothetical cloud of proto-comets at a
great distance from the Sun). During such a passage, the proto-comets
in the Oort cloud would be stirred around. Some tens of thousands
of years later, here on Earth we would notice a dramatic increase
in the the number of comets passing the inner solar system. If
the number of comets increases dramatically, so does the risk of
the Earth colliding with the nucleus of one of those comets.

When examining the Earth's geological
record, it appears that about once every 30 million years a mass
extinction of life on Earth has occurred. The most well-known of
those mass extinctions is of course the dinosaur extinction some
75 million years ago. About 15 million years from now it's time
for the next mass extinction, according to this hypothesis.

This hypothetical "death companion"
of the Sun was suggested in 1985 by Daniel P. Whitmire and John J.
Matese, Univ of Southern Lousiana. It has even received a name: Nemesis
(Greek goddess for revenge, daughter of the Nyx = night).
One awkward fact of the Nemesis hypothesis is that there is no
evidence whatever of a companion star of the Sun. It need not be
very bright or very massive, a star much smaller and dimmer than
the Sun would suffice, even a brown or a black dwarf (a planet-like
body insufficiently massive to start "burning hydrogen"
like a star). It is possible that this star already exists in one
of the catalogues of dim stars without anyone having noted
something peculiar, namely the enormous apparent motion of that
star against the background of more distant stars (i.e. its
parallax). If it should be found, few will doubt that it is the
primary cause of periodic mass extinctions on Earth.

But this is also a notion of mythical power.
If an anthropologist of a previous generation had heard such a
story from his informants, the resulting scholary tome would
doubtless use words like 'primitive' or 'pre-scientific'.
Consider this story:

There is another Sun in the sky, a
Demon Sun we cannot see. Long ago, even before great
grandmother's time, the Demon Sun attacked our Sun. Comets
fell, and a terrible winter overtook the Earth. Almost all
life was destroyed. The Demon Sun has attacked many times
before. It will attack again.

This is why some scientists thought this
Nemesis theory was a joke when they first heard of it -- an
invisible Sun attacking the Earth with comets sounds like
delusion or myth. It deserves an additional dollop of skeptiscism
for that reason: we are always in danger of deceiving ourselves.
But even if the theory is speculative, it's serious and
respectable, because its main idea is testable: you find the star
and examine its properties.

However, since the examination of the
entire sky in the far IR by IRAS with no "Nemesis"
found, the existence of "Nemesis" is not very likely.